1. Field
Various features relate to a small form factor magnetic shield for magnetorestrictive random access memory (MRAM).
2. Background
Magnetoresistive random access memory (MRAM) is a memory technology that stores data using magnetic storage elements and/or cells. FIG. 1 conceptually illustrates a die/wafer that includes an MRAM cell array for storing data. Specifically, FIG. 1 conceptually illustrates a die 100 that includes a substrate 102, several metal and dielectric layers 104 and a MRAM cell array 106. The MRAM cell array 106 includes several MRAM cells 106a-f. Each of these cells includes a magnetic tunnel junction (MTJ). The MTJ is what allows the MRAM to store data.
FIG. 2 illustrates a magnetic tunnel junction (MTJ) 200 of at least one of the cells of FIG. 1. As shown in FIG. 2, the MTJ 200 includes a fixed magnetic layer 202, an insulation layer 204, and a free magnetic layer 206. The magnetic layers 202 and 206 are ferromagnetic layers and the insulation layer 204 is a dielectric layer. Each magnetic layer 202 and 206 has a polarity (a north pole and a south pole). The fixed magnetic layer 202 is fixed because the polarity of the magnetic layer 202 cannot be changed. The free magnetic layer 206 is free because the polarity of the magnetic layer 206 can be changed (the poles can be changed). As mentioned above, the MTJ 200 is what allows the MRAM 200 to store data. The MTJ 200 can have two states. In one state, the free magnetic layer 206 is polarized in the same direction as the fixed magnetic layer 202. In another state, the free magnetic layer 206 is polarized in the opposite direction of the fixed magnetic layer 202.
As described above, the MTJ 200 may be in two possible states, a low resistance state and a high resistance state, which are illustrated in FIGS. 3A-3B and 4A-4B. FIG. 3A illustrates the MTJ 200 in a low resistance state. As shown in FIG. 3A, in a low resistance state, the polarities of magnetic layers 202 and 206 of the MTJ 200 are aligned (the north and south poles of the magnetic layers are on the same side). FIG. 3B illustrates the MTJ 200 in a high resistance state. As shown in FIG. 3B, in a high resistance state, the polarities of the magnetic layers 202 and 206 of the MTJ 200 are opposite to each other (the north pole of the one the magnetic layer is on the opposite side of the north pole of the other magnetic layer).
FIGS. 3A-3B show that the difference between the two states of the MTJ 200 is the polarity of free magnetic layer 206. The difference between the two states of the MTJ 200 may be expressed by the resistance of the MTJ 200 to a current. When the polarities of the two magnetic layers 202 and 206 are aligned, as shown in FIG. 3, the resistance of the MTJ 200 is low. In contrast, when the polarities of the two magnetic layers 202 and 206 are opposite to each other, the resistance of the MTJ 200 is high (relative to the resistance of the MTJ 200 when the polarities of the magnetic layers are aligned). In other words, the resistance of the MTJ 200 is higher when the polarities of the magnetic layers are opposite to each other then when the polarities of the magnetic layer are aligned. These low and high resistance states may correspond to the binary memory states of 0 and 1.
FIGS. 3A-3B illustrates parallel MTJs. However, in some implementations, an MTJ may also be a perpendicular MTJ, as illustrated in FIGS. 4A-4B. As shown in FIG. 4A, in a low resistance state, the polarities of magnetic layers 202 and 206 of the MTJ 200 are aligned in the same direction (the north and south poles of the magnetic layers are in the same direction). FIG. 4B illustrates the MTJ 200 in a high resistance state. As shown in FIG. 4B, in a high resistance state, the polarities of the magnetic layers 202 and 206 of the MTJ 200 are aligned in opposite directions.
As mentioned above, the polarity of a free magnetic layer may be switched. In one instance, the polarity of the free magnetic layer is switched by applying a sufficiently large current through the MTJ. Applying a current in the opposite direction through the MTJ will switch the polarity of the free magnetic layer back. In the case of a STT-MRAM, a spin polarized current may be applied to the MTJ to switch the polarity of the free magnetic layer. A spin polarized current is a current that includes electrons that spin in one direction more than in the other direction (more than 50% spin-up or spin-down). A current is typically unpolarized, but can be made a spin polarized current by passing the current through a magnetic layer.
In another instance, applying a sufficiently large magnetic field will also switch the polarity of the free magnetic layer. Similarly, applying a sufficiently large magnetic field in the opposite direction will switch the polarity of the free magnetic layer back. Thus, in addition to current, magnetic field properties must be taken into account when designing and testing MTJs or any memory that uses MTJs, such as an MRAM. Each cell (i.e., each MTJ) of an MRAM may have different properties (e.g., magnetic properties). That is, each cell may switch back and forth between states under different magnetic field strengths.
One major drawback of an MRAM is that a sufficiently large magnetic field may switch the state of the cells of the MRAM, thereby causing the wrong state to be stored in some or all of the cells in the MRAM. Therefore, there is a need for a method and structure to prevent magnetic fields from affecting the MRAM. More specifically, there is a need for a method and structure to prevent magnetic fields from switching the states of cells of an MRAM. Ideally, any such structure will have a small form factor.